The present invention relates to improvements of a finder for a camera of the type in which an image pick-up system and a finder system are separately installed, such as a lens shutter camera and a video camera.
In this type of camera, an inverse Galilean finder in which an object is viewed by a virtual image has been widely used.
With the trend of reducing the size of a camera body, the camera using a zoom lens of high variable power in the image pick-up system has been proposed. In the camera, a zoom finder of high variable power and with less variation of aberration due to the zooming operation is required for the finder system.
For the above reasons, a finder of a real image type has recently become known, which has a brighter field of view and a higher magnification than the inverse Galilean finder, and can clearly recognize a field frame.
A conventional real image type finder is described in Japanese Patent Unexamined Publication Nos. 61-156018 and 64-65519, for example.
In the finder described in the above-mentioned publications, a chromatic aberration is unsatisfactorily compensated for, so that undesired colors will become more remarkable at larger magnifications. It is therefore a problem to simply increase the magnification.
The following measures are possible to compensate for the chromatic aberrations by using a glass lens. A first measure is to use optical material of high index of refraction and low dispersion. A second measure is to use an achromatic lens consisting of a cheap positive lens element made of optical material of low index of refraction and low dispersion and a negative lens element made of optical material of high dispersion.
In the former measure, cost to manufacture is increased. In the latter measure, the refractive power of the positive lens is increased. The radius of curvature of the face of lens becomes small. This brings about disadvantages, e.g., total reflection occurs on the rear face of the lens and aberrations are very sensitive to an error of mounting the lens to a lens frame.
Also, in view of the compactness, it is preferable to reduce the overall length of the finder system as much as possible. However, since the finder system having a short physical length with a high zoom ratio has increased powers of both the objective and ocular systems, increased chromatic aberration and spherical aberration occur.
In particular, in case of the zoom finder, if the overall length is decreased, the amount of movement of the moving groups is decreased. Accordingly, it is necessary to increase a refractive power of the moving groups in order to increase the zoom ratio. As a result, the chromatic aberration and spherical aberration would become noticeably worse.